1. Technical Field
This application relates to computer storage devices, and more particularly to the field of transferring data between storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
The storage device may contain volatile memory used internally to provide, for example, cache functionality, global sharing of data, task queuing, etc. Although the volatile memory may not be used for permanent storage of data in the storage device, there may be times during the operation of the storage device when the volatile memory contains the most recent, and sometimes the only, copy of certain data. For example, if the volatile memory is used for caching, then it is possible that data written by a host processor to the storage device is initially contained only in the volatile memory prior to being destaged to one or more of the disk drives. Accordingly, it is desirable to guard against the possibility of memory hardware failures.
One way to guard against memory hardware failures is to provide mirrored memory, where data is duplicated in two separate memory locations, preferably corresponding to different memory hardware. Each of the memory locations is a mirror of the other. However, a difficulty introduced by mirrored memory is that it increases the complexity of the software that reads and writes data from and to the mirrored memory. In addition, when a one of the mirrors corresponding to the mirrored memory fails, it becomes even more complex for software that accesses the memory to adapt to the failure and/or adapt to any repairs made to the memory.
It is desirable to address some of the difficulties associated with using mirrored memory.